Download AN1694: The Four Basic Building Blocks of an Op Amp

Application Note 1694
Author: Tamara Schmitz
The Four Basic Building Blocks of an Op Amp
Operational amplifiers are the main building block of analog
systems. They can provide gain, buffering, filtering, mixing and
multiple mathematic functions. In system diagrams, they are
represented by a triangle with 5 connections: positive supply,
negative supply, positive input, negative input and output
(Figure 1). The supply pins are used for powering the device.
They might be connected to ± 5V, for example, or with special
consideration, be connected to +10V and ground. The
relationship between the outputs and the inputs is
straightforward:
(EQ. 1)
Vout = A (Vin+ ‐ Vin‐)
Which means that the output voltage is equal to the gain (A) of
the amplifier times the difference of the input voltages.
Many engineers never need to know what happens inside this
magical triangle. Have you ever been curious? There are four
basic sections: bias, 2:1, gain and buffer (Figure 2). These four
stages can be combined in some op amp devices, but the four
functions are fundamental.
Bias Section
The bias section provides all of the voltages and currents
needed by the other 3 sections. When you dive into this
section, you find circuits like bandgaps and current mirrors.
Bandgaps are small circuits that provide a constant voltage.
Although that may seem like an easy task, the voltage must be
constant even if the supply voltage changes or if the
temperature changes. These effects can be larger than you
expect, so they must be planned for and canceled out.
Current mirrors take a given current and create copies of it for
other circuits. By sizing the transistors or placing multiple
mirrors in parallel, these currents can be properly sized as well.
This powerful technique allows the op amp designer to scale
the amount of current into each stage. The bias section often
consumes a very small fraction of the total current while the
buffer stage consumes the most on-demand current to drive
an output load.
2:1 Section
Given that an op amp has two inputs and one output, there has
to be a 2:1 stage somewhere inside. This may be very close to
the input or later in the signal path. In the simplest form, the
circuit that performs this action is called a differential pair. A
differential pair is two transistors hooked to a common current
source, as in Figure 3. The operation of this circuit is much like
a seesaw. On a seesaw, either side can adjust the balance. If
I'm on one side and you are on the other, I can raise, which
causes your side to lower. If instead I drop my side, then your
side raises. If we correlate height of each side of the seesaw
with the amount of current that flows through each side of the
differential pair, you've got it! In this way, the non-inverting
input controls one side of the circuit and the inverting input
controls the other side of the circuit. Whichever side has a
larger input voltage will have larger current.
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Let's think about what happens when both sides have the
same voltage. Remember the seesaw? If you and I exert the
same amount of pressure (weight) on both sides of the
seesaw, who's side will be higher? Neither! They will be the
same height. What happens if we both apply more pressure? If
we both apply the same amount of pressure, then the seesaw
stays balanced. The only way the seesaw will move is if one of
us applies more pressure than the other. Another way of
looking at this is to say that the seesaw will only respond to
differences in pressure. That is exactly the way the inputs of
the differential pair (and the inputs of the op amp) work.
Achieving the 2:1 function is easier than you might expect. In
the differential circuit shown in Figure 3, there are two inputs
and two outputs. A 2:1 circuit would only have one output. Ok,
then let's ignore one of the outputs. That may seem crude, but
it's been done many times. There are extra transistors you can
add to the unused side if you want to fold and sum the currents
back together, but they aren't necessary. It all depends on
what you are optimizing for. For example, if you are optimizing
for space or simplicity, then there is no need for the extra
circuitry.
The critical parameter of this section is offset voltage. Back to
the seesaw; what if I installed a comfy chair on my side? You
would have to add pressure/weight to your side (without the
comfy chair) just to balance the seesaw. The same can occur
with the transistors. Offset appears if the two transistors
composing the differential pair aren't exactly the same size or
shape. Designers do their best to create exactly duplicate
transistors, but their efforts can be limited by available space
or even the structure of the equipment used to fabricate the
integrated circuit.
Gain Section
Gain means the output is bigger than the input. If your
investments gain, you have more money. If your investments
go down, you lose money. Since an op amp's primary role is to
provide gain, this is the flagship section of the device. In an
ideal op amp, the gain is infinite. That means that the smallest
difference in the input voltages causes the output to slam up
or slam down. Luckily, the power supplies limit how far the
output can travel, so the output doesn't have to reach
thousands or millions of volts in the positive or negative
direction. In the simplest form, the gain function can be
provided by a single transistor. If a single transistor can provide
gain, why do we need to collect many of them to create an op
amp? A single transistor has one input and one output. The op
amp needs two balanced inputs and more gain than a single
transistor can provide.
Since this section has the largest amount of gain, it is also the
section where we would include compensation. Compensation
in an op amp has nothing to do with your paycheck.
Compensation means creating stability. Let me offer a bit of
intuition.
With lots of gain, a small input signal will cause a very large
output signal. Similarly, if the input voltage changes from zero
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Application Note 1694
to small, the output would have to change from zero to large. The
larger the gain or the faster the input changes, the faster we
need the output to respond. Fast circuits need more control.
Think about all the feedback and support that goes into driving a
race car at 200mph. There's an entire pit crew needed. Now
think about what is needed to drive a golf cart. It's much easier
and takes much less skill to drive a golf cart. Going back to the
circuit from this analogy, the race car would need a lot of
compensation to keep it stable (keep it from crashing) while the
golf cart would need very little, if any compensation.
Compensation in op amps takes the form of a capacitance,
shown in Figure 2 connected between the input and output of the
gain stage.
Buffer Stage
The buffer stage serves two critical functions. First, it protects the
gain circuit from anything that can be connected to the output.
Secondly, it drives the load. That means it can provide the current
needed to take the load to the correct output voltage. This stage
provides no gain, which means that the input voltage and the
output voltage are the same size. Instead, it provides more
current, which translates into more power. Think of it like training
in the U.S. Army. A colonel could go and fight with all of his skills,
but if he teaches those skills to an entire battalion, the force of
the group is far stronger. This is very similar to the action of a
buffer. The output tracks the input to the best of its ability with
the force of a larger current, making its Army strong.
output signal to adjust for offsets needed to turn on transistors.
There are many other output stage circuits. Some include
transformers, some use transmission lines and some use
clocked signals. All have been optimized for specific applications.
There is a plethora of information available if you want to learn
more.
Bringing It All Together
What is an op amp? It's a seesaw on a race car followed by an
army battalion fueled by the nourishment of the bias section (I
wouldn't suggest putting that on a T-shirt or sharing this with one
of your company's technical fellows). Truly, it is a unique device
that provides so many wonderful functions. The balanced input
creates the perfect opportunity to allow for feedback. Feedback
will allow us to control gain, to trade it for bandwidth and
performance and to stability systems.
For now, let us celebrate the four main blocks that make the op
amp a pillar of the analog community. Whether you use 5
transistors or 500, an appreciation and understanding of those
internal circuits empowers you to find new ways to use the
greatest analog building block, the op amp. To find a useful
parametric search tool for different kind of op amps, go to
http://www.intersil.com/amps/.
When examining output stages, you will find different classes: A,
AB, B, C and more down the alphabet. Class A is the simplest
stage with the most linearity, but also burns the most power.
Class B and C are related, each slicing away different parts of the
FIGURE 1. THE COMMON INPUT AND OUTPUT PINS OF AN OP AMP
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FIGURE 2. THE FOUR INTERNAL BLOCKS OF AN OP AMP
FIGURE 3. SIMPLE 2:1 STAGE
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